Motor vehicles having longitudinal control systems are well known. Most of the currently available longitudinal control systems control the speed of the motor vehicle by setting an acceleration value at a preset desired or theoretical speed. Additionally, longitudinal control systems enhanced by distance control, which are known as distance-related longitudinal control systems, are also obtainable today from some manufacturers. Such systems, offered, for example by BMW, the Assignee of the present invention, under the name of “Active Cruise Control”, make it possible to automatically drive the motor vehicle while maintaining a desired distance from the vehicle ahead, at a desired speed or at an appropriately slower speed. In principle, the generally known longitudinal control or vehicle speed control systems, which maintain a specific predetermined speed, are enhanced by an additional distance function, such that the use of an “active” driving speed control of this type is also possible in heavy motorway and non-motorway road traffic. The system is therefore generally able to adjust the vehicle's speed to suit the surrounding traffic conditions.
The Active Cruise Control System maintains a desired or set speed when the driver's lane is clear. Under the clear lane condition, a clear drive acceleration value is set. A distance sensor system on the motor vehicle, operating, for example, with a radar, detects a target object or vehicle ahead in the host vehicle's lane. The speed of the host vehicle is adjusted, by the system, to the speed of the motor vehicle or target object ahead, for example, by initiating a suitable following acceleration value, such that a distance control contained in the Active Cruise Control System or in the corresponding longitudinal control system automatically maintains a preset distance, appropriate for the situation, from the motor vehicle or target object ahead. Distance-controlled longitudinal control systems of this type are generally only active or operable beyond a minimum speed of, for example 30 km/h. However, these systems can be enhanced by a “stop-and-go function”, such that distance-related longitudinal control is possible up to and out of standstill.
If, during the control process, at a preset distance from a target object ahead at relatively high speeds, the loss of a target object is detected, the distance-related longitudinal control systems are generally configured such that they immediately switch over from follow control operation into clear drive control (i.e. into the control made for reaching the preset speed). However, this is a problem when the detected loss of the target object is only temporary, because the target object ahead is travelling around a bend, or the loss of the target object is detected due to a sensor error.
DE 100 47 746 A1 discloses a method for distance-related longitudinal control in which, following the loss of a target object and using the information of a navigation system, a probability of the vehicle or target object ahead and/or of the host motor vehicle travelling around a bend is determined. If the determined probability exceeds a predetermined threshold, a time-delayed clear drive acceleration to the preset speed is carried out.
DE 100 06 403 A1 discloses a method for controlling speed and distance in which, and in the event of losing a target object, a transition acceleration value is calculated. Therefore, the vehicle's speed would be a function of the speed of the target object before it was lost, of the driving speed of the motor vehicle and of the distance from the original target object until a new target object is detected. Additionally, the vehicle's speed could be a function until the motor vehicle has reached the location where the target object was lost. From this time, either the usual control following the new target object starts or a clear drive control starts.
A problem exists in reproducing controllable vehicle behaviour when a target object has been detected as being lost at low speeds (stop-and-go) and with resulting very short distances from the original target objects. Attempts to address this problem, in the event of the loss of a target object at very low speeds, for example below 30 km/h, have the entire distance-related longitudinal control system switched off.